Frequency selective tuned circuits are often used before and after RF amplifying stages of a television tuner. Current practice involves the use of a double tuned filter in the signal path before a mixer to further improve frequency selection and to provide better image rejection.
It is desirable to drive the mixer in an optimum manner in order to maximize conversion efficiency, and reduce unwanted harmonics and image frequencies. The mixer of the present embodiment is a Gilbert cell mixer wherein the input circuit is arranged in a common base configuration. Such an arrangement provides a very consistent low input impedance with optimum gain, wide bandwidth, good stability and low noise when the signal input is driven from a balanced source.
To this end, it is desirable to drive the mixer from a balanced source and to maintain this balanced drive when bandswitching between frequency bands.